Support string index for list in get_param

Accept for string index like '1' for list in get_param function
For example, with lengths equal to [4,5,6]. We can write down something
like this: `length: {get_param: [lengths,'1']}`.
Closes-Bug: #1566321

Change-Id: I52447dea93ff2e6a32d597a632093661feed7c71

heat/engine/hot/functions.py

@@ -80,6 +80,14 @@ class GetParam(function.Function):
                 raise TypeError(_('Path components in "%s" '
                                   'must be strings') % self.fn_name)
 
+            if isinstance(collection, collections.Sequence
+                          ) and isinstance(key, six.string_types):
+                try:
+                    key = int(key)
+                except ValueError:
+                    raise TypeError(_("Path components in '%s' "
+                                      "must be a integer "
+                                      "parsable strings.") % self.fn_name)
             return collection[key]
 
         try:

heat/tests/test_hot.py

@@ -1661,6 +1661,10 @@ class StackParametersTest(common.HeatTestCase):
          dict(params={'list': 'foo,bar'},
               snippet={'properties': {'prop1': {'get_param': ['list', 1]}}},
               expected={'properties': {'prop1': 'bar'}})),
+        ('get_list_attr_string_index',
+         dict(params={'list': 'foo,bar'},
+              snippet={'properties': {'prop1': {'get_param': ['list', '1']}}},
+              expected={'properties': {'prop1': 'bar'}})),
         ('get_flat_dict_attr',
          dict(params={'flat_dict':
                       {'key1': 'val1', 'key2': 'val2', 'key3': 'val3'}},